Nickel-aluminum alloy coatings



3,046,205 NICKEL-ALUMINUM ALLOY COATINGS Dwight E. Couch, Washington,D.C., Harold Shapiro,

Hyattsville, and Abner Brenner, Chevy Chase, Md., and

Jean H. Connor, Washington, D.C., assignors to the United States ofAmerica as represented by the Secretary of the Navy No Drawing. FiiedJuly 23, 1959, Ser. No. 829,156 2 Claims. (Cl. 204-37) (Granted underTitle 35, US. Code (1952), see. 266) The invention described herein maybe manufactured and used by or for the Government of the United Statesof America for governmental purposes without the payment of anyroyalties thereon or therefor.

This invention is a continuation-in-part of copending application SerialNo. 665,616, filed June 13, 1957, and relates generally to methods ofproducing a corrosion and oxidation resistant coating that is uniform inthickness, can be produced in any desired thickness and has no free orunalloyed aluminum therein. Such coatings can be produced on metallicsurfaces for increasing wear resistance of sliding or reciprocatingparts of machines and the like, such as bearing surfaces, pistons,piston rings, cylinder Walls and gun mechanisms. More particularly, theinvention pertains to coatings on base metals of nickel alurninum alloysand methods of applying these coatings so that a layer of unalloyednickel remains between the nickel-aluminum layer and the base.

In extensive use at present on metal surfaces subject to sliding rictionare coatings of chromium. This type of coating, while useful in manyrespects, has been found lacking in uniformity of metal distribution andthe metal loses its room-temperature hardness if heated to 1000 C.

Nickel-aluminum alloys are known to have desirable properties for use ashard, strong, corrosion-resistant structural materials or castings.

Objects of the invention, therefore, are to develop methods forcontrollably depositing aluminum on a nickel surface thereby permittingalloying of the aluminum deposit with the nickel surface wherebyunalloyed nickel will remain beneath the layer of nickel aluminide whichis formed, to provide an electrolytically deposited metal coating ofimproved hardness, to provide a coating which will retain itsroom-temperature hardness after heating up to 1000 C., and cooling, andto provide a coating more corrosion resistant than either nickel,aluminum or chromium which, when applied to a base metal, will bedistributed uniformly over the entire base metal surface.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description.

Nickel-aluminum intermetallic compounds have been found to possess manydesirable properties over the separate forming metals, among the chiefof which are a substantial increase in melting point, stability at hightemperatures, good modulus-of-rupture strength, excellent oxidationresistance and hardness. With these desirable properties, however,nickel aluminide, in massive form, is diflicult to fabricate and toobrittle for practical applications. To overcome these difiiculties forcoating use, the nickel aluminide is formed directly on the base metalsurface after the necessary shaping operations have been completed. Thisis accomplished by electrodeposition process steps and several modes ofaccomplishing this result will now be described.

In the first method, nickel is deposited on the base metal in theconventional Way, as by electrodeposition from baths containing nickelsulfate or nickel chloride. If the nickel when deposited is severelysoiled, degreasing of the article with subsequent pumice scrubbingthereof is required prior to aluminum plating. Unless the nickel ismarkedly soiled no precleaning is necessary. Aluminum is then depositedon the nickel plate by electrodeposition from a fused salt bathconsisting of an aluminum halide and an alkali halide, as, for example,aluminum chloride and sodium chloride in the ratio of upwards of 1.5moles of aluminum chloride to 1 mole of sodium chloride. Satisfactoryperformance results from operation at l to 4 amperes/sq. dm. and at atemperature in the range of 200 C. Once prepared the bath must be keptmolten, for if allowed to solidify it is very difficult to remeltwithout breaking the vessel containing the bath. The time of treatmentvaries depending on the thickness of plating desired but it mostcommonly varies between 5 minutes and 1 hour. Very good, coherentaluminum deposits up to 0.8 mil thick can be produced by this methodusing aluminum (2S) anodes. When thicker deposits are desired smoothdeposits of aluminum up to 0.002 inch thick can be obtained by usingtungsten anodes.

After the aluminum has been deposited on the nickel coat the article isdried and placed in a furnace preferably of the electric type such as ahigh frequency induction or resistance furnace. Heating may be conductedin air or in an inert atmosphere, such as in helium or nitrogen, at atemperature in the range from 550 to 750 C. for periods up to about 20hours. At this temperature all the aluminum alloys with the nickel toform a nickel aluminum alloy layer over the unalloyed portion of thenickel layer. This arrangement is accomplished by limiting the depositof aluminum to that of a layer having a thickness less than thethickness of the nickel layer. Actually, the optimum deposit thicknessesare an aluminum deposit approximately onefourth the thickness of thenickel layer whereby all the aluminum is alloyed with the nickel forminga layer of nickel aluminides exceeding in thickness the prior aluminumdeposit but still leaving a portion (about one-half) of the originalnickel layer in unalloyed condition.

Actually, it has been found that the oxidizing effect of heatin in airis beneficial in that there is a marked increase in resistance to saltspray corrosion. In case the alloy is formed by heating in an inertatmosphere the oxidation will in any event occur during normalsubsequent high temperature applications.

The first method, as above described, is particularly useful Where thethickness of the aluminum deposit is less than 0.001 inch or Where theobject being coated might be damaged by heating to too high atemperature. However, as stated above thicker deposits can be obtainedby the use of tungsten anodes.

In the second method aluminum is electrically deposited on the nickelcoat from a fused salt bath operated at a temperature of about 660 to1000 C., i.e., above the melting point of aluminum, the alloy beingformed simultaneously with the plating operation due to the heat of thebath. A suitable bath is cryolite at a temperature of 1000 C. orcryolite dissolved in alkali halides such as sodium chloride andpotassium chloride at a temperature ranging from 660 to 900 C. Usingcryolite with the bath temperature at about 1000 C., and a current flowof 50 amperes per square decimeter surface, a flow of 2 to 3 minuteswill produce a coat of about 0.0005 inch thickness. A coat thickness of0.001 to 0.002 inch is produced by a current of 20 to 25 amperes persquare decimeter flowing for 15 to 30 minutes. For deposits over 0.002inch 15 to 20 amperes per square deciineter flowing for about 1 hour arerequired.

In the operation of the potassium chloride, sodium chloride, cryolitebath although the bath may be operated in the temperature range stated(660900 C.), at the higher temperature the graphite anodes oxidizerather rapidly if an inert atmosphere is not used while at the lowertemperature the aluminum diffuses slowly into the nickel. A series ofruns has shown that a temperature range of about 700 to 800 C. gives asatisfactory compromise of these factors. When the bath is used in thistemperature range, aluminum deposits 0.25 mil thick are produced inminutes at a current density of 7 amperes/sq. dm. To produce thickerdeposits of aluminum at a rate not in excess of the rate of diffusion ofthe aluminum into the nickel the following schedule should be employedin using this bath in the optimum temperature range.

Maximum time permissible with- Approximate Current density, amp/dm. outproducing thickness of free aluminum aluminum on the surface, deposit,mils minutes Since the bath used to electrodeposit the minimum isoperated at temperatures higher than the tempering temperature of steel,when the base material is steel or a steel alloy, heat treatment becomesnecessary to restore the temper of this base material after the nickelaluminide layer has been formed.

Samples to be plated were placed in the bath and allowed to hang therefor to seconds to reach bath temperature before the current was turnedon. Optimum concentrations for the chloride-cryolite bath are: sodiumchloride 440 gms., potassium chloride 560 gms. and cryolite 150 gms.Optimum operating conditions exist at a current density of 2 to 10amps/sq. dm. and in the temperature range of 700-800 C.

In order to avoid producing excess unalloyed aluminum on the surface itis often advantageous to employ varying current densities. Thus, in caseof a chloride-cryolite bath operated at 700 C. deposits of 1 mil thickcan be prepared by plating at 7 amp/sq. dm. for 5 minutes and thenreducing the current density to 34 amp/sq. dm. and plating for to 40minutes. Deposits of 1.8 mils can be produced in two hours by thefollowing schedule: 7 amp/sq. dm., 5 minutes; 4 amp/sq. dm., 20 minutes;2 amp./ sq. dm., 95 minutes. These plating rates can be increased if thebath temperature is raised above 700 C.

An excessive rate of deposition of aluminum is to be avoided since atbath temperature this aluminum is molten and nickel dissolves in moltenaluminum. The problem created is that of depletion of nickel from thenickel layer since the nickel goes into solution in the molten aluminumand the aluminum collects into beads which subsequently flow into thebath carrying the dissolved nickel along. This results in a reduction inthe amount of nickel available for alloying.

This is in fact one of the basic disadvantages of the hotdip processknown in the prior art. As an indication, some 4" x 6" samples wereplated with about 0.001 inch of nickel and accurately weighed. Thesesamples were then sent to a commercial firm for the deposition thereonof aluminum by the hot-dip process. All samples actually lost weight asa result of the dipping process as indicated by the following:

Wt. of

Wt. of nickel base coated (gins) base Wt. of Al deposit (gins) Thenickel aluminide, as produced by the described methods, has a hardnessvarying from 700 to 1050 Vickers at room temperature depending on thepercentage of aluminum in the given nickel aluminide. This fact makesthis compound particularly useful for wear resistance In pistons, pistonrings, cylinder walls, gun mechanism, sliding parts of machine and thelike. In addition, the room temperature hardness of the compound is notaffected by heating to 1000 C. whereas, in the case of chromium thehardness is destroyed.

Further, the nickel aluminide is markedly oxidation resistant even at1100 C. there being only slight surface oxidation. This oxidationresistance is apparent when comparison is made with nickel in which,over a 400 hour period, there is a gain in weight at 1100 C. of 0.05gram per cm. as compared with an 0.01 gram gain for nickel aluminide atthe same temperature and for the same time period. Since nickel andchromium have approximately the same oxidation properties, it isapparent that the oxidation resistance of nickel aluminide exceeds thatof chromium, also. Since the described methods all involveclectrodeposition, the coating thickness may be readily controlled,values ranging from 0.0002 to over 0.002 inch being readily obtainable.

In addition to the beneficial results already set forth the results ofsalt spray corrosion tests (set forth below) on a series of steel alloypanels coated with nickel aluminum alloy clearly indicate the excellentprotection afforded in this respect by the present invention. Further,these tests indicate the superior nature of coatings in which a layer ofnickel remains between the base and the nickel aluminide layer.

Percentage of surface covered Thickness of Thickness of with rust afterNi in inches Al in inches a minimum of 75 hrs. of salt spray 12%. over50%.

less ihan 1%.

None.

1 Oxidized.

While any of the structural metals may be used as the base material, incommon use for this purpose are steel, nickel or copper.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. A method of coating an article which comprises the steps in sequenceof electrolytically depositing in a controlled manner a layer of nickelover the surface of the base metal of said article, electrolyticallydepositing a layer of aluminum from .0001 to .0018 inch thick over saidnickel layer in such manner as to be about one-fourth the thickness ofsaid nickel layer, heating the coated article at a temperature in excessof 550 whereby said aluminum alloys With less than the total thicknessof said nickel layer thereby providing said article with a layer ofnickel covered by a protective surface layer of nickel aluminide, saidlayer of nickel aluminide being free of contamination from the basemetal.

2. The method according to claim 1 wherein the heating step is conductedin an inert atmosphere at a temperature in the range from 550 to 750 C.

References Cited in the file of this patent UNITED STATES PATENTSArmstrong Mar. 7, 1939 Nachtman June 25, 1946 Renzoni Nov. 16, 1948Rhodes Oct. 13, 1953 Whitfield et a1. June 29, 1954 Hansgirg May 24,1955 Lewiston July 14, 1959

1. A METHOD OF COATING AN ARTICLE WHICH COMPRISES THE STEPS IN SEQUENCEOF ELECTROLYTICALLY DEPOSITING IN A CONTROLLED MANNER A LAYER OF NICKELOVER THE SURFACE OF THE BASE METAL OF SAID ARTICLE, ELECTROLYTICALLYDEPOSITING A LAYER OF ALUMINUM FROM .0001 TO .0018 INCH THICK OVER SAIDNICKLE LAYER IN SUCH MANNER AS TO BE ABOUT ONE-FOURTH THE THICKNESS OFSAID NICKEL LAYER, HEATING THE COATED ARTICLE AT A TEMPERATURE IN EXCESSOF 550* WHEREBY SAID ALUMINUM ALLOYS WITH LESS THAN THE TOTAL THICKNESSOF SAID NICKEL LAYER THEREBY PROVIDING SAID ARTICLE WITH A LAYER OFNICKEL COVERED BY A PROTECTIVE SURFACE LAYER OF NICKEL ALUMINIDE, SAIDLAYEER OF NICKEL ALUMINIDE BEING FREE OF CONTAMINATION FROM THE BASEMETAL.